Design, Synthesis, and Characterization of Porous Metal-Organic Materials
Porous metal-organic materials (MOMs) are assembled through coordination between two types of building units, metal or metal-containing nodes and organic linkers. Metal-organic frameworks (MOFs) have 3-D infinite structures and are especially known for high porosity and enormous surface area, leading to diverse applications such as selective gas separation, gas storage and catalysis. In contrast, metal-organic polygons/polyhedra (MOPs) as discrete molecular coordination assemblies are soluble in certain solvents, allowing us to study their solution-chemistry.
In the first project, a microporous MOF with 1-dimensional (1D) bridging helical chain secondary building units (SBUs) shows facile transition from micro- to mesoporosity upon activation conditions. The quickly activated MOF shows permanent microporosity while the slow removal of coordinated aqua ligand results in formation of the mesopores in the microporous MOF.
Second, a strategy to introduce not only the functional groups but also functionalized meso-cavities into microporous MOFs through metal-ligand-fragment coassembly has been studied. With this functionalization, the interior of the MOFs can be tuned by a wide range of functional groups on the ligand fragments, including polar and ionic ones. Depending on the functional groups on the ligand fragments, the introduced cavities can be extended to mesopores in a controllable manner.
Third, a MOF constructed from dicopper paddlewheels and a predesigned ligand bearing carboxylate, pyridine, and amide groups enables selective adsorption of CO2 over CH4 and high H2 adsorption. The cooperative catalytic activity in a tandem one-pot deacetalization-Knoevenagel condensation was demonstrated.
In the fourth and fifth section, an optically and thermally switchable azobenzene was introduced into a MOF and MOPs, respectively. The freshly synthesized MOF adsorbed a significant amount of CO2. Upon light irradiation, the adsorbed gas molecules were squeezed out of the MOF due to the change of conformation of the azobenzene groups inside the pores. The adsorbent returned to its original state when allowed to stay with gentle heating. In addition, solubility of srMOPs was optically controlled by trans-cis isomerization of the azobenzene moieties. Interestingly, guest molecules were trapped during cis to trans isomerization and released in the trans to cis conversion. This srMOP can be applied to uses requiring stimuli responsive capture and release of guest molecules, such as in controlled drug delivery systems.
Finally, an organic linker with multiple conformations was used to synthesize both single and core-shell molecular squares, whose formations were controlled by reaction temperatures. Intriguingly the core-shell structure assembly was successfully employed as a template to prepare a heterobimetallic assembly, in which the metal substitution occurred exclusively in the core. This work might pave the way for the exploration of enzyme-mimicking molecular catalysts.